In medical infusion, the flow typically needs to be restricted to rather low rates, such as, for example, 1000 microlitres per hour. Delivery of liquids at flow rates of a few millilitres per hour or less may be realized by connecting a source of pressurized liquid to a capillary of small internal diameter. In capillaries the rate of flow through the capillary has a well-defined relation to the length and diameter of the capillary, and to the difference in pressure between the capillary inlet and the capillary outlet. For any given pressure difference the flow rate may thus be fixed at a desired value by choosing a capillary of suitable length and diameter.
A problem with capillaries of very small internal diameter (microcapillaries) is that bubbles of gas in the liquid may have a serious impact on the pressure difference or pressure drop required to drive a given flow rate through the capillary, and in the worst case bubbles may lead to an effective blocking of the capillary. This is due to the phenomenon of fragmentation of a (larger) bubble at the inlet of the capillary into a plurality of small bubbles within the capillary. The small bubbles are separated from each other by plugs of liquid, and each small bubble requires a certain pressure difference between its ends to move along the capillary. That pressure difference is largely independent of bubble length. Bubble fragmentation at the inlet may fill the capillary with so many small bubbles that the pressure difference available for generating liquid flow is reduced or fully consumed by the sum of pressure drops needed to drive the small bubbles along the capillary. Therefore, flow through the capillary may be severely reduced or even stopped by bubble fragmentation.
Fused silica microcapillaries with an internal diameter of 10 to 100 micrometers are widely used in the field of chemical analysis, in applications such as capillary electrophoresis and gas chromatography. Microcapillary flow restrictors for use in medical infusion are easily made by cutting suitable lengths (a few centimeters each) off from fused silica microcapillary stock. Other choices of material are also available, such as polymeric capillaries or micromachined planar capillary structures.
Unfortunately, however, experience shows that the occurrence of bubble fragmentation in microcapillary flow restrictors is not predictable. Out of 100 flow restrictors made, some may have a very low tendency towards bubble fragmentation whereas others will fragment virtually any bubble that enters. There is a lack of yield and a lack of predictability. Both are major obstacles in the industrial use of microcapillary flow restrictors, for example in mass fabrication of medical infusion devices.